Facilitating cells (FCs) from mouse have been described. See, for example, U.S. Pat. No. 5,772,994. FCs are not stem cells, but significantly improve the initial and long-term engraftment of stem cells. For example, transplantation of stem cells alone only prolongs survival of a transplant patient, while the presence of FCs result in sustained and long-term HSC engraftment. See, also, Kaufman et al., 2005, J. Exp. Med., 201:373-83).